1. Field of the Invention
This invention relates to an objective lens suitable for use in liquid such as water, and particularly to a photo-taking lens for an underwater single-lens reflex camera.
2. Related Background Art
In recent years, the number of people who attempt to take underwater photographs have been increasing with an increase in diving population. In order to meet this demand, cameras for underwater photographing and waterproof housings for enabling ordinary cameras to be used underwater are being studied.
It is usual that a photo-taking lens designed for exclusive use for underwater photographing is mounted on or contained in a camera exclusively for underwater photographing, and lenses therefor are known, for example, from applicant's Japanese Laid-Open Patent Application No. 59-13210 and Japanese Laid-Open Patent Application No. 64-31113. These lenses are designed to display their highest performances underwater and therefore, high descriptive power can be expected of them in underwater photographing.
On the other hand, a method of putting an ordinary camera into a waterproof housing and taking underwater photographs has the merit of being capable of intactly using an ordinary photo-taking lens (a photo-taking lens designed for use in the air), but this is a kind of expedient and therefore has been very unsatisfactory in convenience of use as well as in performance.
Now, the recent studies of zoom lenses for still cameras have produced excellent results in downsizing, widening of angle, heightening of magnification and reduction of cost. The recent compact and light-weight zoom lenses which have swept away the image of the former zoom lenses which have been large and heavy and unsatisfactory in performance have rapidly infiltrated into users from their convenience and have been popularized. There is even the impression that this explosive spread of zoom lenses has reduced conventional single-focus lenses to "special lenses" for use by some high-level users.
However, turning out attention to lenses for underwater photographing, single-focus lenses are still the mainstream and there has been no zoom lens for underwater photographing. A greater potential demand is expected of underwater zoom lenses than of ordinary lenses for use in the air. Yet underwater zoom lenses have scarcely been studied.
Of course, according to the afore described waterproof housings, it is possible that zoom lenses for use in the air are used under water. However, this poses a more serious problem than the use of single-focus lenses, as will hereinafter be described.
In a waterproof housing, there are two kinds of so-called ports attached to the front of a photo-taking lens and serving as a water-resisting window, i.e., a plane port comprised of a plane parallel plate and a dome port comprised of a lens substantially concentric with the entrance pupil of the photo-taking lens. However, if an underwater photograph is taken by the plane port, the occurrence of great positive distortion and chromatic difference of magnification is unavoidable in principle. On the other hand, when the dome port is resorted to, distortion and chromatic difference of magnification do not occur, yet great positive curvature of image field occurs in conformity with the curvature of the concentric lens. Further, the dome port has refracting power conforming to the curvature thereof and therefore, when it is combined with a zoom lens, great focus movement may occur due to a variation in the focal length of the zoom lens. To decrease such focus movement and curvature of image field, the curvature of the dome port can be weakened, but if it is done, the waterproof housing will unavoidably become impractically bulky.